Lets see, I just netted a pair of old BC-639A radio receivers for a song. They look OK, one is gonna become a parts set for the other, so parts might not be that big a deal.

One of them is marked with a painted-on tag that says: Modified for SAGE, could this be the giant old NORAD SAGE computer system ? If so, that's a bit of trivia for it. The SAGE tagged on is modified a bit, with a fat old capacitor sitting on the topside of the chassis, terminating in an octal socket that is soldered to the chassis. Could this be part of the SAGE modifications ? The socket soldering job doesn't look that good, I might take it out, and then re-do it with more care to the soldering. In any case, the other 639 was at one time or another the basis for a project, and wasn't completed. So chances are good that I will try to save the SAGE one, even though it is much dirtier than its relative, thus more restoration work ! The power supply is long gone, but it looks like it takes relatively simple input voltages, so finding a PS circuit might not be that hard.

But that's OK, it will be a fun project. I would like to know, if there are any folks out there that have one of these, or have had the chance to use one, and knows how good they are or are not. It would be greatly appreciated to know whether or not it is a radio worth restoring. ( Since I am a fan of the old SAGE system, even if this radio was not modified for use under that program, it will be cool to have around anyway ! ).

I have two of those receivers, up and working. The tunable range is 100-156 mc. AM. Their selectivity is about as wide as a barn door and I mainly listened to aircraft chatter on them.
Didn't think much of them as they've been sitting in a relay rack in my barn going on ten years now. I believe I also have the companion BC-639 signal generator sitting in the same rack as well.
I think they're more of a curiosity than anything else.

_________________"Resistance may be futile but capacitance has potential"

OK, interesting ! I didn't think that they would be anything for performance, but a working one is a neat oddity I suppose. Makes for an interesting conversation piece huh ? I've never seen the matching signal generator, but I suppose they might be out there.

Looking around inside, I assume that one could build a power supply inside, but I'm not sure if that would louse up the radio's performance, after all, it is a VHF receiver. . .

Since they're AM, I would guess that hearing the broadcast band in that area would be out of the question, unless one could figure a simple mod to do inside it, then you'd have an oddity that received FM radio !

The innards are basically an SCR-522 modified for tunable operation. Other than being a bit wide, they are actually a pretty good receiver.

There was a mod performed by the military to add FM, and I understand those usually had blue-painted front panels. Also to add squelch.

You probably know that they were a standard control tower receiver in their day, and they usually also contain the red British Air Ministry tags with the crown symbol, since that is where the design came from.

The companion transmitter was the BC-640, a huge rack cabinet affair full of 3C24 tubes which CQ magazine once said "generates TVI even when turned off."

Yup, I knew a couple of those things ! One of mine fortunately has the red tag, while the parts set doesn't.

I didn't know about the mod for FM, and since neither of mine are blue, or have squelch, I will assume that the one was somebody's basis for a project, while the other could have been, and by the tag, was likely modified to some degree for use in an installation where the SAGE computer system was somehow connected. Could have been stuffed into a radar installation connected to SAGE, hence the tag. . .

Was told what you mentioned there, about the set being a control tower receiver for their time, also heard somewhere that some were picked up surplus after WW2, and converted using unknown conversions to a state in which they could be used in civilian installations. I really don't see what they would have to convert, if they wanted to make it "non-military", , all they would have to do is remove the name tags, and spray paint the front panel and the case ! Oh yes, if you were really picky, you could scratch out the paper schematic that is deteriorating in mine, so it might go anyway. Then you'd probably have a civilian BC-639 A, hehe.

I heard about that 640, wow. Interesting, but probably best relegated to life as a museum piece !

I have heard they were also used in Korea at Forward Air Controller bases.

But stock up on .01uF capacitors! They must have close to a dozen and a half Micamolds in them, and I can guarantee that they will all be leaky, if not downright shorted by now. I had to change every single one of them out in my set that appeared to have never been used in service.
Curt

Ooooh man, ok ! Damn, counting inside mine here, yeah, there are a bunch ! Well, time to dig out the Radio Amateurs Handbook, and find the page for decoding the Mica 6-dot color code !

It's kinda interesting, the one that I will be relegating to a parts set is very clean inside, but the one I want to save, the SAGE one, , is a mess ! But that's good, me having to clean it will aid in learning about the radio, and will also be a good patience tester !

I gotta remove the dial gear assembly, to clean it, and by the looks of it, I might have to remove one of the tuning caps to get to the other little coupling thing screw, .

BAMA has the manual. Also Fair Radio Sales sells a print copy. Mine has in internal PS mod. The original was a separate rack mounted rectifier unit. They're kinda finicky to align. You have to bend loops a precise distance from each other. The sensitivity of mine isn't up to snuff. I keep finding carbon comp resistors going south. It also fades after a while , probably more resistors or a lightweight hombrew PS. Yes, still a working work in progress.

You can pick up FM broadcasts, but the selectivity is rather narrow for that and it sounds like crap. Just tune to either side of the null.

OK on the 73 Curt, I edited accordingly ! Guess I'll be collecting .01 uf caps for this thing, ! Thankfully that value is common as dirt !

Thanks Kent ! I didn't know about Fair having a print copy, might be good to look into. . . I recall a friend that had one mentioning something about them being tough to align, could be interesting with those loops inside.

You mentioned yours had some resistors going out, so I might consider replacing some of those too. Would an internal PS in this unit be a good idea to attempt ? Or should I just build a little one to sit behind it, and leave it the way it is ? I think for originality, that would probably be the better alternative.

Years ago i had one my dad got.Iwent thru the thing and aligned it .Was ok bit wide heard aircraft on it too.Once it warmed up it did not drift much at all.I found it easy to work on.Mine was green with red tags.Found using the right antenna helped it a lot.I used a aircraft ant on it ,picked up lots of stuff.PS was home brew and was in a old wooden radio cabinet.I sold it all manual,ect for 45.00 back in 1997.Angelo

When I first fired my set up, I used a healthy sized dynamotor. I let it run for a couple hours and you could tell the dynamotor was slowing down due to increased load as the capacitors went bad.

I was running static tests on it, not trying to pick up signals, but after I would clip out one capacitor the dynamotor would then regain its speed and repeat the process. Those capacitors were popping like pop corn!

What amazed me is that my set was one that never was issued and used. It proved to me that those Micamold capacitors go bad just sitting there.

It was fun watching the voltages on the tube grids and plates jump around to the tune of the dynamotor. I knew in no time what capacitor had failed doing that.
Curt

Interesting on the dynamotor Curt, now I get the feeling that I might end up having to replace ALL the mica caps in there, WOW !

Oh well, learning project again ! Probably would be safest to do that anyway. That also surprises me, I would think that micas should be OK, especially in a new radio ! But, results may very, as your experience is testament to !

Neat Jim ! If you do get yours out of storage, and onto the bench, please show us some pics of it, and or the signal gen, would certainly be neat ! Good luck with your in the event that it does need some work,

Tom, great find. I have three of the BC 639 receivers on the "haven't got it yet shelf". Two of the BC 638 signal generators
on the shelf below them and two NOS BC 640 transmitters standing beside the shelf.

Curt's first advise to me when I contacted him about them, was to watch out for those pesky .01 caps all lined up in a row.

He was right. the first one I opened up had micamold square paper caps in it. They are leaky.

But in the second one they were still micamold square, but they were mica caps. I took a couple apart to verify. It may have been a later version. but they still need to checked.

They were also used during WWII as fighter base control in multiple sets.

Still looking to find the rectifier RA 42 and the PE 100 dynamotor

There is also a rumor out there about a rack mounted speaker made by Rolo. Only a few sightings of that one though.

Cool ! Glad to hear that there are several folks out there that own these odd little sets, I'm not alone !

Mine are the A variety, and since one is going to donate itself for the hopeful life of the other one, yeah, only one is going to be up and running. Not sure how hard these things are to find, but I would presume that because of their band coverage, they are probably not too popular of a boat anchor !

Interesting on the other WW2 use, I didn't know that ! I don't know about the speaker deal, but there does seem to be a power supply and signal generator for these things, in which all three of them would make for a pretty rare setup.

The problem is that people tend to think that those Micamold capacitors are mica, when for the most part, they are only paper capacitors. Micamold is only the Brand name of the manufacturer who made them. And Micamold made paper as well as mica capacitors.

Actually, the way it is supposed to be, but I have seen many times when this did not apply, was that the first dot on a six dot capacitor should be black for mica and white if it is an AWS (American War Standard) paper capacitor. That is assuming that we are talking military equipment, as there were too many different standards to go by back then. Still confuses the heck out of me at times.
Curt

I think I heard somewhere about the Micamold being a paper cap, but what surprises me is that they are supposed to be fairly reliable ! In short, replacing all of them would probably yield the best overall results !!!

Not sure about the dot colors, but since there were zillions of different standards, it is certainly grounds for confusion of a restorer !

In any case, it would appear that this radio is a good candidate for learning about and discovering the most effective ways to replace old Micamolds, !

One interesting thing about the paper Micamolds is that when you remove one, try reading it with a voltmeter. I have found that they have mutated into little batteries and actually produce voltage!

Maybe they are good for something these days after all - - a lifetime replacement bias battery...

So many standards - prewar, RMA, JAN, RETMA... they should all be listed on the back page of the Surplus Conversion Handbook, the big yellow one which just has the photos and schematics without text. But then not everyone has that book.

Collins bought huge quantities of Micamold mica capacitors throughout the early 1950's, there are lots of them in the 51J series. And those fail more often than Cornell-Dubilier or other makes, too.

Micamold also made power resistors in the same housing as the paper capacitors, but with a light brown color. And we mustn't forget that novice style transmitter kit they offered, which I think may have been first marketed before there actually was a novice class license. I think one of the forum members has one of those transmitters.

I have a BC-639A I received from a local estate. I handle a lot of vintage electronics but am really green with this type of receiver. My question is, how do you open the darn thing? There was a little rattling inside and I wanted to inspect the innards and take photos of the inside before I put it up for sale. I removed all screws around the shell but it wouldn’t budge. I then removed some screws on the face and it must have loosened something inside because it fell inside and is now banging around. Now the front screws will not go back into place, obviously since they weren’t holding the face in place but parts inside. Now I’m stuck with a beautiful piece but now with a bunch of metal rolling around inside and I still can’t get into it. I can provide pics if anyone out there wants to help out a frustrated member